This invention relates to paper substitutes composed of nonwoven polymeric fibers or filaments and more particularly to a fabric or laminate which can be printed by conventional methods and further processed into useful items such as envelopes.
Spunbonded fabrics are distinguished from other nonwoven fabrics by an integrated and continuous manufacturing procedure comprising the steps of spinning, laydown, consolidation, and bonding of continuous filaments to form a fabric, typically a fabric having planar-isotropic properties due to the random laydown of the filaments. Some of the processes are described in U.S. Pat. Nos. 3,338,992, 3,991,244, 4,017,580, 3,502,763 and No. 4,405,297. The composition of spunbonded fabrics have included polyester, nylon, polyethylene and polypropylene.
Most spunbonded fabrics have not been proposed as paper substitutes for a variety of reasons, including difficulties in providing a variety of colors and printing on the surface of the fabric, and difficulties in processing the fabric into useful articles such as envelopes. A notable exception is spunbonded polyethylene sold under the trademark "TYVEK". These fabrics are flash spunbonded and have a filament size of in the order of 0.05 denier. As a result, the material has the appearance of a white sheet, which may be printed by conventional methods.
There are several disadvantages in the use of spunbonded polyethylene or "TYVEK"sheets as paper substitutes. The material is relatively expensive compared to paper, and more difficult to print and handle. When processed into envelopes or other similar products, the products are relatively limp, and a separate coloring or printing process must be employed to impart color to the sheet.
Another drawback with "TYVEK" envelopes is the absence of reliable security in the sealed envelope. The end flap employs an active adhesive covered by a strip of a release sheet. After sealing, the flap can be opened and re-closed without any evidence of tampering.
Other envelope materials have been proposed but have received limited acceptance. These include paper and plastic laminates and plastic film pouches.
It would be desirable to employ spunbonded polypropylene as a paper substitute. Isotactic polypropylene is the most widely used polymer for spunbonded production, and because of its low density, provides the highest yield at the lowest cost. Polypropylene fabrics also offer excellent physical properties in terms of strength, stiffness and tear resistance. One such fabric is sold under the trademark "TYPAR". The fabrics, however, are porous and cannot be printed or dyed by conventional methods. Also, the fabrics cannot be processed in conventional paper handling and processing equipment due to the fibrous surfaces. As a result, the use of such fabrics has been confined to areas such as carpet backing, furniture, bedding, and geotextiles, and none of these fabrics have been successfully converted into paper substitute products, such as envelopes.